Atrial fibrillation is one of the most common cardiac arrhythmia. Atrial fibrillation is a debilitating disease that afflicts 20 million people worldwide. Health consequences associated with atrial fibrillation include decreased cardiac output, less regular ventricular rhythm, the formation of blood clots in the atrial appendages, and an increased incidence of stroke. While some drugs are available for the treatment of atrial fibrillation, they have a number of side effects which reduce their therapeutic utility. The use of atrial counter shocks remains one of the primary treatments for atrial fibrillation.
Anatomically, the heart includes a fibrous skeleton, valves, the trunks of the aorta, the pulmonary artery, and the muscle masses of the cardiac chambers (i.e., right and left atria and right and left ventricles). The driving force for the flow of blood in the heart comes from the active contraction of the cardiac muscle. This contraction can be detected as an electrical signal.
The beginning of a cardiac cycle is initiated by a P wave, which is normally a small positive wave in the body surface electrocardiogram. The P wave induces depolarization of the atria of the heart. The P wave is followed by a cardiac cycle portion which is substantially constant with a time interval on the order of 120 milliseconds (“ms”).
The “QRS complex” of the cardiac cycle occurs after the substantially constant portion. The dominating feature of the QRS complex is the R wave which is a rapid positive deflection. The R wave generally has an amplitude greater than any other wave of the cardiac cycle, and has a spiked shape of relatively short duration with a sharp rise, a peak amplitude, and a sharp decline. The R wave is the depolarization of the ventricles and therefore, as used herein, the term “ventricle activations” denotes R waves of the cardiac cycle. The QRS complex is completed by the S wave, which is typically a small negative deflection that then returns the cardiac signal to baseline.
Following the S wave, the T wave occurs after a delay of about 250 ms. The T wave is relatively long in duration (e.g., about 150 ms). The cardiac cycle between the S wave and the T wave is commonly referred to as the ST segment. The T wave is a sensitive part of the cardiac cycle, during which an atrial defibrillation shock is to be avoided, in order to reduce the possibility of induced (and often fatal) ventricular fibrillation. The next cardiac cycle begins with the next P wave. The typical duration of a complete cardiac cycle is on the order of about 800 ms.
Unlike patients afflicted with ventricular fibrillation, patients afflicted with atrial fibrillation are conscious. The pain associated with the administration of the defibrillation shock can be severe, and there is a need to reduce the pain to the patient being treated while maintaining clinical efficacy of the defibrillation shock. Other fibrillation-based conditions in which pain may be experienced by the patient include shock treatment of hemodynamically stable ventricular tachycardia.